Optical signal-processing apparatus, receiving apparatus, and optical network system

ABSTRACT

An optical modulator combines and inputs a signal light propagating through the optical network and a control light having information concerning the optical network to a nonlinear optical medium. The optical modulator modulates the signal light according to changes in intensity of the control light, in the nonlinear optical medium.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuing application, filed under 35 U.S.C.§111(a), of International Application PCT/JP2009/068703, filed on Oct.30, 2009.

FIELD

The embodiments discussed herein are related to an opticalsignal-processing apparatus that sequentially superposes and multiplexesinformation onto a signal light, a receiving apparatus, and an opticalnetwork system.

BACKGROUND

It is necessary for a future optical network to perform, for example,processing, such as branching and inserting of a signal light andswitching of the same, at an apparatus, such as a repeating opticalnode, which is placed at a location remote from a terminal apparatus,while using a conventional optical communication system as a basis. Indoing the above, it is effective from a viewpoint of energy efficiencyto propagate and process information such that as little conversion aspossible is performed between an optical signal and an electric signal.

However, an existing repeating optical node or the like performs signalprocessing by photoelectric conversion as performed in a terminalapparatus, and for example, a transmitted signal light is once convertedto an electric signal, and this electric signal is electricallyprocessed, whereafter the processed electric signal is converted to anoptical signal again. This complicates the apparatus configuration, andfurther necessitates large electric power to perform photoelectricconversion.

By the way, in an optical network, various kinds of information aremonitored at various points on a real-time basis, and effective networkcontrol is performed based on the monitored information. In a futureoptical network, the amount of such information increases, and hence itbecomes effective to realize an energy-saving optical network. Further,to realize a more flexible optical network, a function is effective inwhich information is inserted into the network not only at an opticalnode, but also at a desired point.

However, at present, insertion of information is performed at an opticalnode apparatus or a terminal apparatus, and particularly, monitoredinformation is propagated e.g. by performing photoelectric conversion ofa signal light and writing the information into a header part of thesignal light, or by using a dedicated optical wave.

Note that a transmission technique is known in which in a relay station,which is disposed between a transmitting station and a receiving stationvia an optical transmission path, there are provided a phase conjugatelight-generating device that has signal light/pump light-supplying meansfor supplying a signal light input from the transmitting station and apump light to a nonlinear optical medium, and signal light/phaseconjugate light-extracting means for extracting an output signal lightgenerated by modulating the input signal light by the pump light, and aphase conjugate light, using the input signal light and the pump lightsupplied to the nonlinear optical medium, and modulation means formodulating the pump light based on monitored data specific to the relaystation, wherein the signal light containing the modulated monitoreddata and the phase conjugate light are transmitted to the receivingapparatus station (e.g. see Japanese Patent No. 3436310).

As described above, the conventional optical network has a problem thatdue to photoelectric conversion performed on a signal light, a largepower loss is caused by insertion of another signal or propagation ofinformation.

SUMMARY

In one aspect of the embodiments, there is provided an opticalsignal-processing apparatus including: an optical modulator configuredto combine and input a first signal light and a second signal lighthaving information to a nonlinear optical medium, and modulate the firstsignal light according to changes in intensity of the second signallight, in the nonlinear optical medium.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an optical signal-processing apparatus according to afirst embodiment;

FIG. 2 illustrates an optical signal-processing apparatus according to asecond embodiment;

FIG. 3 illustrates an optical signal-processing apparatus according to athird embodiment;

FIG. 4 illustrates an optical signal-processing apparatus according to afourth embodiment;

FIG. 5 illustrates an optical signal-processing apparatus according to afifth embodiment;

FIG. 6 illustrates an optical signal-processing apparatus according to asixth embodiment;

FIG. 7 illustrates an optical signal-processing apparatus according to aseventh embodiment;

FIG. 8 illustrates an optical signal-processing apparatus according toan eighth embodiment;

FIG. 9 illustrates another example of modulation of a control lightperformed in the optical signal-processing apparatus;

FIG. 10 illustrates a receiving apparatus according to a ninthembodiment;

FIG. 11 illustrates an example of a receiving apparatus that demodulatesan optical signal modulated by an RF carrier wave;

FIG. 12 illustrates a receiving apparatus according to a tenthembodiment;

FIG. 13 illustrates a receiving apparatus according to an eleventhembodiment;

FIG. 14 illustrates an optical network according to a twelfthembodiment;

FIG. 15 illustrates an example of a case where optical modulation isperformed using frequency-division multiplexing;

FIG. 16 illustrates an example of application of the opticalsignal-processing apparatus;

FIG. 17 illustrates an example of application of the opticalsignal-processing apparatus; and

FIG. 18 illustrates an optical signal-processing apparatus according toa thirteenth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereafter, a first embodiment will be described in detail with referenceto a drawing.

FIG. 1 illustrates an optical signal-processing apparatus according to afirst embodiment. As illustrated in FIG. 1, the opticalsignal-processing apparatus includes an optical modulator 1.

A signal light E_(S) having a wavelength λ_(s) and a control lightE_(Ct) having a wavelength λ_(Ct) are input to the optical modulator 1.The signal light E_(S) is a signal light propagating through an opticalnetwork. The signal light E_(S) is e.g. a continuous wave (CW) light ora signal light having data signal. The control light E_(Ct) is e.g. asignal light having information concerning the data signal or theoptical network. The information is e.g. information on a data signalinserted into the network at a repeating optical node, information foroperating and managing optical network apparatuses forming the opticalnetwork, and information on monitored images, and temperature, pressure,electric power, etc.

The optical modulator 1 has a nonlinear optical medium. The opticalmodulator 1 combines and inputs the input signal light E_(S) and thecontrol light E_(Ct) to the nonlinear optical medium thereof. Theoptical modulator 1 modulates the signal light E_(S) by the controllight E_(Ct) in nonlinear optical medium, and outputs a modulated lightE_(O) having a wavelength λ_(S) and modulated based on informationcontained in the control light E_(Ct).

Let it be assumed, for example, that the signal light E_(s) and thecontrol light E_(Ct) having respective waveforms illustrated in FIG. 1are input to the optical modulator 1. In this case, the signal lightE_(S) is intensity-modulated using the control light E_(Ct) by theoptical modulator 1, as illustrated in the waveform of the modulatedlight E_(O) in FIG. 1. That is, the optical signal-processing apparatusis capable of superposing the information contained in the control lightE_(CT) on the signal light E_(S) propagating through the optical networkwithout performing photoelectric conversion of the signal light E_(S).

The optical signal-processing apparatus thus modulates the signal lightE_(S) having the wavelength λ_(S) according to the information containedin the control light E_(Ct) having the wavelength λ_(Ct), in thenonlinear optical medium. This makes it possible to superpose theinformation contained in the control light E_(Ct) on the signal lightE_(S) without performing photoelectric conversion of the signal lightE_(S), which makes it possible to reduce power loss.

Further, this makes it possible to insert information into the opticalnetwork at a desired point within the optical network to transmit theinformation therethrough, receive the inserted information aftertransmission, and recognize the information as the information from thedesired point. Further, in a case where the information is e.g.monitored information, it is not necessary, for example, to use anothercommunication network, such as a wireless network, to propagate theinformation.

Note that when a signal is superposed as illustrated in FIG. 1, theoptical modulator 1 modulates the signal light E_(S) by the controllight E_(Ct) without adversely affecting the data signal in the signallight E_(S).

Next, a second embodiment will be described in detail with reference toa drawing. In the second embodiment, a description will be given of anexample in which a WDM (Wavelength Division Multiplexing) signal lightis modulated by the control light.

FIG. 2 illustrates an optical signal-processing apparatus according tothe second embodiment. As illustrated in FIG. 2, the opticalsignal-processing apparatus includes an optical modulator 11.

Input to the optical modulator 11 are an N-channel WDM signal lightincluding signal lights E_(S1), E_(S2), . . . , E_(SN) having respectivewavelengths λ_(S1), λ_(S2), . . . , λ_(SN), and a control light E_(Ct)having a wavelength λ_(Ct).

The optical modulator 11 has a nonlinear optical medium. The opticalmodulator 11 combines and inputs the input WDM signal light and thecontrol light E_(Ct) to the nonlinear optical medium. The opticalmodulator 11 modulates the WDM signal light by the control light E_(Ct)in the nonlinear optical medium, and outputs modulated lights E_(O1),E_(O2), . . . , E_(ON) having respective wavelengths λ_(S1), λ_(S2), . .. , λ_(SN) (WDM signal light), modulated based on the informationcontained in the control light E_(Ct). In other words, the opticalmodulator 11 outputs the WDM signal light as the modulated lightsE_(O1), E_(O2), . . . , E_(ON) on each of which the informationcontained in the control light E_(Ct) is superposed.

As described above, the optical signal-processing apparatus is capableof modulating the WDM signal light by the control light E_(Ct).

Next, a third embodiment will be described in detail with reference to adrawing. In the third embodiment, a description will be given of anexample in which the WDM signal light modulated in the second embodimentis split by an optical splitter.

FIG. 3 illustrates an optical signal-processing apparatus according tothe third embodiment. As illustrated in FIG. 3, the opticalsignal-processing apparatus includes the optical modulator 11 and theoptical splitter 21. Note that the optical modulator 11 is the same asthe optical modulator illustrated in FIG. 2, and description thereof isomitted.

The optical splitter 21 splits a WDM signal light modulated by theoptical modulator 11 into modulated lights E_(O1), E_(O2), . . . ,E_(ON), having respective wavelengths λ_(S1), λ_(S2), . . . , λ_(SN),and outputs the split modulated lights.

That is, in the optical signal-processing apparatus illustrated in FIG.3, the WDM signal light modulated by the optical modulator 11 is splitby the optical splitter 21 into the modulated lights having therespective wavelengths, and the modulated lights are output therefrom.This makes it possible to match with optical systems having differentwavelength bands.

Next, a fourth embodiment will be described in detail with reference toa drawing. In the fourth embodiment, a description will be given of anexample in which a signal light is modulated using a control light byoptical parametric amplification using an optical fiber.

FIG. 4 illustrates an optical signal-processing apparatus according tothe fourth embodiment. As illustrated in FIG. 4, the opticalsignal-processing apparatus includes an optical coupler 31, an opticalfiber 32, and an optical filter 33.

The signal light E_(S) having a wavelength λ_(S), and the control lightE_(Ct) having a wavelength λ_(Ct) and an optical power P_(Ct) are inputto the optical coupler 31. The optical coupler 31 combines and outputsthe signal light E_(S) and the control light E_(Ct) to the optical fiber32. As the optical coupler 31, there may be used e.g. a WDM coupler. TheWDM coupler has a small transmission loss, and is capable of combiningand splitting the control light E_(Ct) and the signal light E_(S) almostwithout affecting the signal light E_(S).

The optical fiber 32 uses the control light E_(Ct) as pump light andmodulates the signal light E_(S) according to changes in intensity ofthe optical power P_(Ct) of the control light E_(Ct) and outputs themodulated light E_(O) having the wavelength λ_(S). As the optical powerP_(Ct) of the control light E_(Ct) is increased, the signal light E_(S)is optical parametrically amplified by four-wave mixing (FWM) in theoptical fiber 32. This makes it possible to amplify and modulate thesignal light E_(S) according to the information contained in the controllight E_(Ct) (e.g. 0, 1).

The optical filter 33 is an optical filter for blocking the controllight E_(Ct) and passing the signal light E_(S). As the optical filter33, there may be used, for example, an optical bandpass filter, a bandblocking filter that blocks wavelength components other than the signallight E_(S), or a WDM optical coupler (used in a state in which theinput-output direction thereof is opposite to that of the opticalcoupler 31).

As described above, the optical signal-processing apparatus modulatesthe signal light E_(S) having the wavelength λ_(S) according to changesin intensity of the control light E_(Ct) having the wavelength λ_(Ct),in the optical fiber 32. This makes it possible to superpose informationcontained in the control light E_(Ct) on the signal light E_(S) havingthe wavelength λ_(S) without performing photoelectric conversion of thesignal light E_(S).

Note that optical parametric amplification selectively occurs withrespect to the signal light E_(S) having the same polarization componentas that in the control light E_(Ct). Therefore, the signal light E_(S)and the control light E_(Ct) may be controlled to be in the optimumpolarization state or in the random polarization state (polarizationscramble), using a polarization controller, or may be caused to operatewith respect to a desired polarization state by a polarization diversityconfiguration.

Further, FWM and optical parametric amplification in the optical fiber32 have response times of femtosecond order. Optical modulation ispossible even at a data speed beyond a terabit, and the operation ispossible without depending on the data speed of the control lightE_(Ct).

Further, the optical signal-processing apparatus illustrated in FIG. 4is also capable of modulating the WDM signal as described with referenceto FIG. 2, and is also capable of splitting the WDM signal modulated bythe optical splitter as described with reference to FIG. 3.

Further, an optical filter for preventing the control light E_(Ct) frombeing output may be disposed downstream of the optical fiber 32. Forexample, a WDM coupler is disposed which splits the signal light E_(S)and the control light E_(Ct) and passes the signal light E_(S) havingthe wavelength λ_(S). This makes it possible to prevent the controllight E_(Ct) from propagating through the optical network.

Next, a fifth embodiment will be described in detail with reference to adrawing. In the fifth embodiment, a description will be given of anexample in which an idler light is modulated.

FIG. 5 illustrates an optical signal-processing apparatus according tothe fifth embodiment. As illustrated in FIG. 5, the opticalsignal-processing apparatus includes an optical coupler 41, an opticalfiber 42, and an optical filter 43.

The optical coupler 41 is the same as the optical coupler 31 describedwith reference to FIG. 4. The optical coupler 41 combines and outputsthe signal light E_(S) and the control light E_(Ct) to the optical fiber42.

The optical fiber 42 outputs an idler light (having a wavelength λ_(I))of the signal light E_(S) generated by FWM, as an intensity-modulatedlight.

The optical filter 43 is an optical filter that passes the idler light(modulated light E_(O)) optically modulated by the signal light E_(S)and the control light E_(Ct). As the optical filter 43, there may beused an optical bandpass filter, a band blocking filter which blockswavelength components other than the idler light E_(I), or a WDM opticalcoupler.

As described above, the optical signal-processing apparatus modulatesthe idler light having the wavelength which is generated in the opticalfiber 42, according to changes in intensity of the control light E_(Ct)having the wavelength λ_(Ct). This makes it possible to superposeinformation contained in the control light E_(Ct) on the idler lighthaving the wavelength λ_(I) without performing photoelectric conversionof the signal light E_(S).

Note that the optical signal-processing apparatus illustrated in FIG. 5is also capable of modulating the WDM signal as described with referenceto FIG. 2, and is also capable of splitting the WDM signal modulated bythe optical splitter as described with reference to FIG. 3.

Hereafter, the optical parametric amplification and the idler light willbe described. The frequencies of the control light E_(Ct), the idlerlight, and the signal light E_(S) are represented by ω_(Ct), ω_(I), andω_(S), respectively. The frequencies ω_(Ct), ω_(I), and ω_(S) satisfythe following Expression (1):ω_(Ct)−ω_(I)=ω_(S)−ω_(Ct)≠0  (1)

Now, the optical fiber is used as the nonlinear optical medium, and thelength of the optical fiber is represented by L while a loss isrepresented by α. Further, it is assumed that in the optical fiber, alllight waves are in the same polarization state, and the input power ofthe control light E_(Ct) sufficiently larger than the optical power ofthe signal light E_(S) and the optical power of the idle light.

When the wavelength λ_(Ct) of the control light E_(Ct) is adjusted tothe zero-dispersion wavelength λ₀ of the optical fiber, by way ofexample, the signal light E_(S) and the idler light output from theoptical fiber are approximately given a gain G_(S) and a gain G_(I),respectively, expressed by the following Expressions (2) and (3):G _(S)=exp(−αL)[1+φ²(L)]  (2)G _(I)=exp(−αL)[φ²(L)]  (3)

Note that φ(L) represents a nonlinear optical phase shift, and is givenby the following Expression (4):φ(L)=γP _(P)(0)l(L)  (4)

Here, P_(P)(0) represents the input power of the control light E_(Ct),and l(L) represents a nonlinear interaction length, and is given by thefollowing Expression (5):l(L)=(1−e ^(−αL))/αL  (5)

Further, γ represents a third-order nonlinear coefficient, and is givenby the following Expression (6):

$\begin{matrix}{\gamma = \frac{\omega\; n_{2}}{{cA}_{eff}}} & (6)\end{matrix}$

Here, n₂ and A_(eff) represent a nonlinear refraction index and aneffective core cross-sectional area within the optical fiber,respectively.

The optical parametric amplification gain G_(S) changes with respect tothe nonlinear coefficient, the input power of the control light E_(Ct)as the pump light, and the magnitude of the interaction length.Particularly, under the condition of λ_(Ct)=λ₀ which provides widewavelength range of constant generation efficiency, the gainapproximately increases by the square of the value of the product asindicated by the Expressions (2) and (3), and the power variationcorresponding to the increase in the gain causes amplitude modulation.

Here, the generation efficiency of the optical parametric effectstrongly depends on the polarization state of the interacting lightwaves. Specifically, when the light waves input to the optical fiber arein the same polarization state, the generation efficiency of the FWMbecomes maximum, whereas when the light waves are in polarization statesorthogonal to each other, the FWM is reduced.

From the Expression (3), the generation efficiency of the idler light isincreased by increasing the optical power of the control light E_(Ct) asthe pump light, and hence by preparing the control light E_(Ct) at ahigh optical power level, it is possible to perform optical intensitymodulation with high efficiency.

As an optical intensity modulator, it is possible to use e.g. aMach-Zehnder interferometer optical fiber switch or a nonlinear opticalloop mirror switch other than the above-mentioned one.

Note that the optical fiber may have a zero-dispersion wavelength on ashorter wavelength side than the wavelength of the control light E_(Ct)as the pump light, and set a value of the product of a chromaticdispersion, the wavelength separation of the control light E_(Ct) andthe signal light E_(S), the value of the nonlinear optical coefficient,the optical power of the control light E_(Ct), and a length of theoptical fiber to achieve the phase matching condition of an opticalparametricamplification.

Next, a sixth embodiment will be described in detail with reference to adrawing. In the sixth embodiment, a description will be given of anexample of optical phase modulation using cross phase modulation (XPM)within an optical fiber. In this case, it is possible to perform phasemodulation with a value corresponding to twice as large as the valuegiven by the Expression (4), with respect to the same control lightpower. In doing this, it is not necessary to set the zero-dispersionwavelength of the optical fiber and the wavelength of the control lightto match each other.

FIG. 6 illustrates an optical signal-processing apparatus according tothe sixth embodiment. As illustrated in FIG. 6, the opticalsignal-processing apparatus includes an optical coupler 51, an opticalfiber 52, and an optical filter 53.

The optical coupler 51 is the same as the optical coupler 31 describedwith reference to FIG. 4. The optical coupler 51 combines and outputsthe signal light E_(S) and the control light E_(Ct) to the optical fiber52.

The control light E_(Ct) and the signal light E_(S) may have respectivepolarization states adjusted such that desired optical modulation can beobtained, and then are input to the optical coupler 51.

The optical fiber 52 changes the phase of the signal light E_(S)according to changes in intensity of the control light E_(Ct) (XPM).That is, the optical fiber 52 gives optical phase modulation accordingto changes in intensity of the control light E_(Ct) to the signal lightE_(S).

For example, as illustrated in the waveform of the modulated light E₀ inFIG. 6, when the optical power P_(Ct) of the control light E_(Ct) issmall, the phase of the signal light E_(S) is equal to φ₁, and when theoptical power P_(Ct) of the control light E_(Ct) is large, the phase ofthe signal light E_(S) is equal to φ₂.

A phase difference in the given phase modulation Δφ=|φ₁−φ₂| isdetermined according to intensity of the control light E_(Ct), anonlinear coefficient, and a length of the optical fiber 52.

The optical filter 53 is the same as the optical filter 33 describedwith reference to FIG. 4. The optical filter 53 extracts and outputs thesignal light E_(S).

As described above, the optical signal-processing apparatus modulatesthe phase of the signal light E₃ having the wavelength λ_(S), by thecontrol light E_(Ct) having the wavelength λ_(Ct), in the optical fiber52. This makes it possible to superpose information contained in thecontrol light E_(Ct) on the signal light E₃ having the wavelength λ_(S)without performing photoelectric conversion of the signal light E_(S).

Note that when the nonlinear optical medium is a third-order orsecond-order nonlinear optical medium, the signal light E_(S) issubjected to optical phase modulation by the optical Kerr effect or theoptical parametric effect of the control light E_(Ct) (pump light) inthe nonlinear optical medium. More specifically, it is possible torealize the optical phase modulation by using the third-order nonlinearoptical medium, such as an optical fiber, or the second-order nonlinearoptical medium, such as a LiNbO₃ (periodically-poled LN) waveguidehaving the quasi phase matching structure.

Further, the optical signal-processing apparatus illustrated in FIG. 6is also capable of modulating the WDM signal as described with referenceto FIG. 2, and is also capable of splitting the modulated WDM signal bythe optical splitter as described with reference to FIG. 3.

Next, a seventh embodiment will be described in detail with reference toa drawing. In the seventh embodiment, a description will be given ofmodulation of a control light by the optical signal-processingapparatus.

FIG. 7 illustrates an optical signal-processing apparatus according tothe seventh embodiment. As illustrated in FIG. 7, the opticalsignal-processing apparatus includes an LD (laser diode) 61.

A control signal B which is information illustrated in FIG. 7 is inputto the LD 61. Here, the control signal B is created by a modulationmethod, such as amplitude modulation, phase modulation, frequencymodulation, or, on an as-needed basis, multilevel modulation. The LD 61outputs the control light E_(Ct) having an optical power P_(Ct) and awavelength λ_(Ct) illustrated in FIG. 7, according to the input controlsignal B. The control light E_(Ct) is output e.g. to the opticalmodulator 1 or 11 illustrated in FIGS. 1 to 3, or the optical coupler31, 41, or 51 illustrated in FIGS. 4 to 6.

As described above, the optical signal-processing apparatus modulatesthe control light E_(Ct) using the control signal B which isinformation. This enables the optical signal-processing apparatus tomodulate the signal light E_(S) by the control light E_(Ct) having theinformation.

Next, an eighth embodiment will be described in detail with reference toa drawing. In the eighth embodiment, a description will be given ofanother example of modulation of the control light by the opticalsignal-processing apparatus.

FIG. 8 illustrates an optical signal-processing apparatus according tothe eighth embodiment. As illustrated in FIG. 8, the opticalsignal-processing apparatus includes a multiplier 71, a local oscillator72, and an LD 73.

The multiplier 71 multiplies the control signal B and a RF (radiofrequency) carrier wave (subcarrier signal) output from the oscillator72. The oscillator 72 outputs e.g. a carrier wave having a frequency off. As a consequence, from the multiplier 71, the control signal B(f) isoutput which is formed by modulating (subcarrier-modulating) the carrierwave having a frequency of f by the information signal.

The LD 73 is driven by modulated current by the control signal B(f)output from the multiplier 71, and accordingly outputs the control lightE_(Ct) having the optical power P_(Ct) and the wavelength λ_(Ct).

That is, the optical carrier is subcarrier-modulated at frequency fusing the information signal B, and the control light E_(Ct) is outputfrom the LD 73. The control light E_(Ct) is output e.g. to the opticalmodulator 1 or 11 illustrated in FIGS. 1 to 3, or the optical coupler31, 41, or 51 illustrated in FIGS. 4 to 6.

As described above, the optical signal-processing apparatus outputs thesubcarrier (RF-carrier) wave modulated by the control signal B which isdata information, in a state superposed on (having modulated) theoptical carrier. This enables the optical signal-processing apparatus tomodulate the signal light E_(S) by the control light E_(Ct) having thesubcarrier-modulated data information with the subcarrier frequency off.

Although the above description has been given of a method of directlymodulating the laser using the LD or 73 as the modulator, by way ofexample, the modulation may be performed using an external modulator fora continuous wave light. Examples of the external modulator include aLiNbO₃ intensity/phase modulator, an electronic absorption (EA)modulator, a semiconductor optical amplifier, and a nonlinear opticalmedium. Further, any of methods including methods of amplitudemodulation, phase modulation, and frequency modulation, can be appliedto the modulation method.

Further, when the signal light E_(S) is a data modulated light, it ispossible, by setting the above-mentioned frequency f to a frequencysufficiently higher than the baseband of the data signal, to prevent thesignal quality from being degraded due to existence of the data signalof the signal light E_(S) and the control signal B in the same frequencyband.

FIG. 9 illustrates another example of modulation of the control lightperformed in the optical signal-processing apparatus. As illustrated inFIG. 9, the optical signal-processing apparatus includes an LD 76 a, alocal oscillator 76 b which outputs a signal having a frequency of f, anoptical modulator 76 c to which the signal having a frequency of f isinput, and an optical modulator 76 d to which the information signal Bis input.

As illustrated in FIG. 9, this example is effective e.g. in a case wheretwo external modulators are used with respect to the frequency f and thecontrol signal B, and a case where relatively high speed information issuperposed using the high frequency f.

Although in the FIG. 6 example, the description has been given of thecase where phase modulation is a binary modulation of φ₁ and φ₂ forsimplicity, when optical subcarrier modulation is performed using theabove-mentioned XPM, the optical modulation, including amplitudemodulation, is analog modulation.

Next, a ninth embodiment will be described in detail with reference to adrawing. In the ninth embodiment, a description will be given of areceiving apparatus that demodulates data information.

FIG. 10 illustrates a receiving apparatus according to the ninthembodiment. As illustrated in FIG. 10, the optical signal-processingapparatus includes a PD (Photo Diode) 81, an amplifier 82, a LPF (LowPass Filter) 83, and a demodulation circuit 84. The receiving apparatusillustrated in FIG. 10 demodulates e.g. the control signal B describedwith reference to FIG. 7.

The signal light E_(S) is input to the PD 81. The PD 81 is an opticalreceiving apparatus which converts the signal light E_(S) to an electricsignal, and the PD 81 outputs e.g. an electric signal having a waveformillustrated in the modulated light E₀ in FIG. 4.

The amplifier 82 amplifies an electric signal output from the PD 81. TheLPF 83 passes a low frequency band of the electric signal amplified bythe amplifier 82. For example, the LPF 83 passes an envelope having awaveform illustrated in the modulated light E₀ in FIG. 4.

The demodulation circuit 84 is e.g. a circuit for demodulating thecontrol signal B according to the modulation method of the controlsignal B. Note that when the control signal B is intensity-modulated,the demodulation circuit 84 is not needed.

As mentioned above, the receiving apparatus is capable of demodulatinginformation (control signal B) from the signal light E_(S).

Note that a digital signal-processing circuit that demodulatesinformation or eliminates erroneous detection, fluctuations, etc. of thedemodulated information may be disposed downstream of the demodulationcircuit 84.

FIG. 11 illustrates an example of a receiving apparatus that demodulatesan optical signal modulated by the subcarrier (RF-carrier) wave, asdescribed with reference to FIG. 8. As illustrated in FIG. 11, thereceiving apparatus includes a PD 86 a, an amplifier 86 b, a BPF(Band-pass filter) 86 c, and a demodulation circuit 86 d.

The receiving apparatus illustrated in FIG. 11 converts a signal lightto an electric signal using the PD 86 a, and transmits the electricsignal through the BPF 86 c which passes the electric signal in a bandaround the frequency f of the sub-carrier signal. Thereafter, thecontrol signal B is demodulated using the demodulation circuit 86 d.

Next, a tenth embodiment as an example of the receiving apparatusillustrated in FIG. 11 will be described in detail with reference to adrawing. In the tenth embodiment, a description will be given of anotherexample of the receiving apparatus that demodulates information.

FIG. 12 illustrates the receiving apparatus according to the tenthembodiment. As illustrated in FIG. 12, the receiving apparatus includesa PD 91, an amplifier 92, a BPF 93, a multiplier 94, and a LPF 95. Thereceiving apparatus illustrated in FIG. 12 demodulates e.g. the controlsignal B described with reference to FIG. 8.

The signal light E_(S) is input to the PD 91. The PD 91 is an opticalreceiving apparatus that converts the signal light E_(S) to an electricsignal, and the PD 91 outputs a control signal B(f) which is obtained,e.g. as described with reference to FIG. 8, by converting(subcarrier-modulating) the optical signal modulated by the sub-carrier(RF-carrier) wave to an electric signal.

The amplifier 92 amplifies the electric signal output from the PD 91.The BPF 93 is a band pass filter which passes the electric signalamplified by the amplifier 92 in a band around a frequency of thesub-carrier signal. The frequency in a main pass band of the BPF 93 isset e.g. to the frequency f of the local oscillator 72 described withreference to FIG. 8.

In the FIG. 12 example, the multiplier 94 forms a square-law detector,and outputs an envelope of a received signal. The LPF 95 passeslow-frequency components of a signal output from the square-lawdetector. For example, the LPF 95 passes signals in a band not higherthan the baseband of the control signal B described with reference toFIG. 8. This makes it possible to obtain e.g. the control signal Bhaving information, described with reference to FIG. 8. Note that as thesquare-law detector, it is possible to use not only the above-mentionedconfiguration, but also an envelope detector such as a half-waverectifier circuit using a resistor, a capacitor, and a coil, or thelike.

As described above, the receiving apparatus is capable of demodulatingthe subcarrier-modulated information (control signal B(f)) from thesignal light F.

Note that in the receiving apparatus in FIG. 12, the local light and thesignal light E_(S) may be combined and then input to the PD 91. It isassumed that the frequency (wavelength) of the local light differs fromthe frequency (wavelength) of the signal light by a desired mistunedfrequency (f_(IF)). This makes it possible to obtain an electric signalin an intermediate frequency band (f_(IF)) from the PD 91.

Further, a digital signal-processing circuit which demodulatesinformation or eliminates an erroneous detection, fluctuations, etc. ofthe demodulated information may be disposed downstream of the LPF 95.

Next, an eleventh embodiment will be described in detail with referenceto a drawing. In the eleventh embodiment, a description will be given ofstill another example of the receiving apparatus that demodulatesinformation.

FIG. 13 illustrates a receiving apparatus according to the eleventhembodiment. In FIG. 13, components identical to those in FIG. 12 aredenoted by identical reference numerals, and description thereof isomitted.

In the receiving apparatus in FIG. 13, a CR (Clock Recovery) circuit 101and a PLL (Phase-lock loop) circuit 102 are provided at one of inputs ofthe square-law detector of the multiplier 94.

The CR circuit 101 generates a clock signal having a frequency of asub-carrier signal, based on an electric signal output from the BPF 93.For example, the CR circuit 101 generates a clock signal having thefrequency f of the local oscillator 72 described with reference to FIG.8.

The PLL circuit 102 synchronizes the phase of an electric signal outputfrom the BPF 93 and to be input to the multiplier 94, with the phase ofa clock signal output from the CR circuit 101.

As described above, the receiving apparatus demodulates thesubcarrier-modulated phase information (control signal B(f)) from thesignal light E_(S) even by a synchronous detector having the CR circuit101 and the PLL circuit 102 connected to one of the inputs thereof.

Next, a twelfth embodiment will be described in detail with reference toa drawing. In the twelfth embodiment, a description will be given of anoptical network to which the optical signal-processing apparatus isapplied.

FIG. 14 illustrates the optical network according to the twelfthembodiment. As illustrated in FIG. 14, the optical network includesoptical fibers 111 a to 111 i, and optical signal-processing apparatuses112 a to 112 e. The optical signal-processing apparatuses 112 a to 112 eare sometimes referred to as the first, . . . , j−1th, jth, j+1th, . . ., and nth optical signal-processing apparatuses from the left side asviewed in FIG. 14, respectively.

The jth optical signal-processing apparatus 112 c includes a multiplier113, a local oscillator 114, a control light source 115, an opticalcombiner 116, an optical splitter 117, and an optical fiber 111 e. Themultiplier 113, the local oscillator 114, and the control light source115 correspond e.g. to the multiplier 71, the local oscillator 72, andthe LD 73 illustrated in FIG. 8, respectively, for example, and detaileddescription thereof is omitted.

The optical combiner 116 combines the control light E_(Ct) output fromthe control light source 115 with the signal light E_(S) propagatingthrough the optical network. The optical splitter 117 splits off thecontrol light E_(ctj) from the signal light E_(S) propagating throughthe optical network. That is, the optical splitter 117 prevents thecontrol light E_(ctj) from propagating through the optical networkdownstream thereof. The optical combiner 116 and the optical splitter117 are e.g. WDM couplers.

The optical signal-processing apparatuses 112 a to 112 e may modulatethe signal light E_(S) by the control light E_(ctj) using part of thelaid optical fibers forming the optical network.

The signal light E_(S) modulated by the control light E_(ctj), thoughnot illustrated in FIG. 14, is demodulated e.g. by the receivingapparatus illustrated in FIG. 11, FIG. 12, or FIG. 13. For example, anoptical coupler for branching the signal lights E_(S) is provided at apredetermined location in the optical network, and the modulated signallight E_(S) is received by the PD 91 illustrated in FIG. 12 or 13 todemodulate the information.

The optical signal-processing apparatuses 112 a, 112 b, 112 d, and 112 eeach include the same multiplier, local oscillator, and control lightsource as those included in the optical signal-processing apparatus 112c. The local oscillator 114 of the jth optical signal-processingapparatus 112 c outputs an oscillation signal having a sub-carrierfrequency of f_(j), and the local oscillators of the other opticalsignal-processing apparatuses output oscillation signals havingrespective sub-carrier frequencies f₁ to f_(n). That is, each opticalsignal-processing apparatus has assigned thereto one of a plurality ofsub-carrier signals different in frequency, and performs opticalmodulation using the control signal B_(j)(f_(j)) formed bysubcarrier-modulating the control signal B_(j).

Therefore, the control light E_(ctj) as the locally generatedinformation which is subcarrier-modulated by the frequency f_(j) (j=1, .. . , or n) is sequentially superposed on the signal light E_(S) havingthe wavelength λ_(S) and propagating through the optical network, andthe receiving apparatus is capable of demodulating the locally generatedinformation contained in the control light E_(ctj) by discriminating thesame according to the frequency.

As described above, the optical signal-processing apparatuses insertedin the optical network superpose respective control lights ofinformation pieces, which are subcarrier-modulated by respectivedifferent frequencies, on the signal light. This enables the receivingapparatus to distinguish and demodulate a plurality of differentinformation pieces contained in respective control lights.

FIG. 15 illustrates an example of a case where optical modulation isperformed using frequency-division multiplexed signal. In FIG. 15,components identical to those in FIG. 14 are denoted by identicalreference numerals, and description thereof is omitted.

In FIG. 15, the optical signal-processing apparatus 112 c includesmultipliers 118 a to 118 k and local oscillators 119 a to 119 k. Thelocal oscillators 119 a to 119 k output carriers having respectivefrequencies f_(j1) to f_(jk). The multipliers 118 a to 118 k have inputthereto control signals B_(j1) to B_(jk) and carriers having frequenciesf_(j1) to f_(jk), respectively, and multiply these signals to output theproduct to the control light source 115.

Although FIG. 14 illustrates the example of modulation using one controlsignal B_(j)(f_(j)) identified by the frequency f_(j) by each opticalsignal-processing apparatus j, optical modulation may be performed usinga frequency-division multiplexed (FDM) signal of sub-carrier signalsgenerated by modulating carrier waves f_(j1), and f_(jk) havingrespective frequencies, by the control signals B_(j1), . . . , andB_(jk), respectively, as illustrated in FIG. 15.

When a plurality of information data items exist at a point whereoptical modulation is to be performed, the information data items aresubjected to FDM, and then the optical modulation is performed, wherebyit is made possible to collectively superpose the plurality ofinformation data items on one optical carrier. The FDM signal hasexcellent matching properties with already developed techniques, such asa microwave technology, and it is possible to use various types ofelectrical signal processing in combination. Particularly, in using anorthogonal FDM (OFDM: Orthogonal Frequency Division Multiplexing)signal, normal signal processing, such as serial parallel conversion,inverse discrete Fourier transform, or parallel serial conversion, isused. Information is obtained in the optical network or another opticalnetwork by extracting a FDM signal and demodulating each sub-carriersignal.

Further, the optical signal-processing apparatus is capable oftransmitting the modulated signal light E_(S) to another opticalnetwork, and the receiving apparatus in the other optical network iscapable of receiving and modulating the signal light E_(S).

FIGS. 16 and 17 illustrate examples of application of the opticalsignal-processing apparatus. In applying the optical signal-processingapparatus, configurations as illustrated in FIGS. 16 and 17 areenvisaged. In FIG. 16, a signal light is propagated from a point Atoward a point B, information pieces 1, . . . , and N are superposed onthe signal light by optical signal-processing apparatuses at desiredintermediate points 1, . . . , and N, respectively, and then thesuperposed signal is received at the point B. It is possible to envisageapplication not only to ordinary information communication but also tocommunication in one direction in bidirectional communication,transmission of monitor information, optical wiring, etc.

On the other hand, in the case of FIG. 17, a signal light is propagatedfrom the point A, information items 1, . . . , and N are superposed onthe signal light by optical signal-processing apparatuses at desiredintermediate points 1, . . . , and N, respectively, and then thesuperposed signal is transmitted up to the point A, where the superposedsignal is received. It is possible to envisage application to collectionand transmission of monitored information within the network, controlinformation, and request information, bidirectional communicationinformation, etc.

Note that in the embodiments illustrated in FIGS. 14 to 17, it is alsopossible to extract and observe information having been superposed onthe signal light E_(S) by an upstream optical signal-processingapparatus, at a desired intermediate point. As an extraction method usedin this case, it is possible to use, for example, a method of tappingpart of the signal light E_(S) by a power branching circuit or the like,or a method of combining a CW light having the same wavelength as thecontrol light E_(Ct) in FIG. 5 with the signal light E_(S), with theconfiguration illustrated in FIG. 5, and extracting a generated idlerlight e.g. by an optical filter or a WDM coupler.

The optical signal-processing apparatus is capable of collectivelysuperposing control signals (information pieces) on the WDM signallight, and hence it is possible to distribute the information piecescontained in the control signals through the network on a real-timebasis. By disposing a short optical fiber which does not affect a signallight at each point j, and disposing a WDM coupler for combining andsplitting off a control light at input and output ends of the opticalfiber, whereby even when data information is sent by the signal light,it is possible to superpose the locally-generated control signal almostwithout affecting the signal light. As the short optical fiber, there isused, specifically, an optical fiber which is several meters to severaltens meters long and causes a nonlinear optical effect, and which hardlygenerates the nonlinear effect in the power of the signal light itself.The control light having a sufficient power to superpose a controlsignal is input to the optical fiber. For example, when superposingamplitude modulation of 0.1% (mark rate of ½) on a signal light, if anoptical fiber having a length of 20 m and a nonlinear coefficient 20(1/W/km) is used, necessary power of the control light is approximatelyequal to 50 mW. An actually needed degree of modulation depends on amethod of modulation to be given (amplitude modulation or phasemodulation), a bit rate of the control signal, detection sensitivity,and so forth.

Alternatively, a monitor signal may be superposed by extracting anappropriate length of a transmission fiber, disposing WDM couplers atfront and rear ends of the fiber, and thereby using the nonlinearoptical effect within the transmission fiber. The nonlinear coefficientof an ordinary transmission fiber is approximately equal to 2 (1/W/km),and hence, in the above-mentioned model, the transmission fiber having alength of approximately several hundreds meters makes it possible torealize monitoring the optical network. In the actual optical network,if it is possible to use part of the transmission fiber as an opticalmodulator as mentioned above, the optical network can be monitoredanywhere within the optical network. Further, in doing this, if nocontrol light is input, the signal light is not affected at all, andmatching properties with conventional systems are excellent.

Particularly, in a case where a medium improved in the nonlinear effectis to be used, there may be employed, as an optical fiber, for example,a highly nonlinear fiber (HNLF) to begin with, and a fiber or waveguideconfiguration in which a nonlinear refractive index is increased bydoping a core with e.g. germanium or bismuth, a fiber or waveguideconfiguration in which an optical intensity is increased by reducing amode field, a fiber or waveguide configuration which uses chalcogenideglass or Bi₂O₃ glass, a photonic crystal fiber or waveguideconfiguration, and so forth. Further, as another nonlinear opticalmedium, there can be also employed a semiconductor optical amplifierhaving a quantum well structure, a quantum dot semiconductor opticalamplifier, a silicon photonics waveguide, InGaAsP photonics waveguide,etc. Further, as still another nonlinear optical medium, it is alsopossible to use a device that generates a second-order nonlinear opticaleffect, such as three-optical-wave mixing. In this case, it is possibleto use e.g. a LiNbO₃ waveguide having a quasi phase matching structure,a GaAlAs element, or a second-order nonlinear optical crystal, for thesedevices. Also in the case of using the second-order nonlinear opticalmedium, a configuration is preferable in which wavelength arrangementenables phase matching.

Next, a thirteenth embodiment will be described in detail with referenceto a drawing. In the thirteenth embodiment, a description will be givenof a feedback process executed by the optical signal-processingapparatus.

FIG. 18 illustrates an optical signal-processing apparatus according tothe thirteenth embodiment. As illustrated in FIG. 18, the opticalsignal-processing apparatus includes an optical modulator 121, a monitorcircuit 122, a comparison circuit 123, a power control circuit 124, apolarization control circuit 125, a polarization controller 126, and anoptical power controller 127.

The optical modulator 121 corresponds e.g. to the optical modulator 1illustrated in FIG. 1.

The monitor circuit 122 monitors the quality of a modulated signal lightoutput from the optical modulator 121. The monitor circuit 122 includese.g. a filter for extracting the wavelength of the modulated signallight, and a light receiving element for receiving a signal lightextracted by the filter.

The comparison circuit 123 calculates operating characteristics ofoptical modulation based on the optical power, a waveform, a spectrum,etc. of the modulated signal light monitored by the monitor circuit 122,and compares the calculated characteristics with predetermined thresholdvalues.

The power control circuit 124 controls the optical power of the controllight and the signal light concerning the optical modulation based onthe results of comparison output from the comparison circuit 123. Forexample, the power control circuit 124 controls the optical powercontroller 127 that controls the optical power of the control light.Also, the power control circuit 124 controls the optical powercontroller that controls a modulated state of the signal light includedin the optical modulator 121.

The polarization control circuit 125 controls polarization states of thecontrol light and the signal light concerning the optical modulationbased on the results of the comparison by the comparison circuit 123.For example, the polarization control circuit 125 controls thepolarization controller 126 that controls the polarization state of thecontrol light. Also, the polarization control circuit 125 controls apolarization controller provided in the optical modulator 121, forcontrolling the polarization state of the signal light.

When necessary, the control light E_(ct) is input to the polarizationcontroller 126. The polarization controller 126 controls thepolarization state of the control light E_(ct) according to the controlby the polarization control circuit 125.

The optical signal-processing apparatus performs the feedback control asdescribed above, whereby it is possible to output a properly modulatedsignal light from the optical modulator 121.

According to the above-described optical signal-processing apparatus, itis possible to reduce the power loss.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatvarious changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical network system comprising: a pluralityof optical signal-processing apparatuses each including an opticalmodulator configured to combine and input a first signal light and asecond signal light having information to a nonlinear optical medium,and modulate the first signal light according to changes in intensity ofthe second signal light, in the nonlinear optical medium, wherein theplurality of optical signal-processing apparatuses each opticallymodulate the first signal light by the second signal light having theinformation, which is a sub-carrier signal having a carrier wavefrequency assigned thereto, and transmit the modulated first signallight to an optical network, and the information is demodulated in theoptical network or another optical network, for each sub-carrier signalhaving the carrier wave frequency, on a subcarrier signal-by-subcarriersignal basis; and wherein the second signal light of the plurality ofoptical signal-processing apparatuses is a frequency-divisionmultiplexed signal of sub-carrier signals having a plurality of carrierfrequencies assigned thereto, the first signal light is opticallymodulated by the second signal light and propagates through an opticalnetwork between the optical signal-processing apparatuses, and in theoptical network or another optical network, the information is extractedas the frequency-division multiplexed signal and is then demodulated ona subcarrier signal-by-subcarrier signal basis.
 2. The optical networksystem according to claim 1, wherein a transmitting station and areceiving station of the first signal light exist at different points.3. The optical network system according to claim 1, wherein atransmitting station and a receiving station of the first signal lightexist at the same point.